power: reset: st: Remove obsolete platforms from dt doc

This patch removes support for STiH415/6 SoC's from the
st-restart dt binding documentation, as support for these
platforms is being removed from the kernel. It also updates
the dt example to a currently supported platform.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <linux-pm@vger.kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>